I. Field of the Invention
This invention relates to apparatus and methods for remote measurement of the surface temperatures of objects, e.g. moving metal sheets in metal fabrication or treatment plants and the like.
II. Description of the Prior Art
In many industries it is necessary to obtain an accurate measurement of the surface temperatures of objects but the nature of those objects, or the manner in which they are handled, makes it difficult or impossible to use temperature measurement means which require direct contact with the objects themselves, e.g. thermocouples or the like. For example, in metal rolling plants, heated metal sheets, rods or bars are often passed rapidly through fabrication equipment so that contact methods of temperature measurement are impractical.
In such cases, optical pyrometry is often used for remote temperature measurement. This involves measuring infra-red radiation emitted from the surface of the object as an indication of the temperature of the surface.
Optical pyrometers normally work by measuring the amount of radiation emitted in a certain spectral band (or bands) from the surface to be measured. The emitted radiation can be described quantitatively by the Stefan-Boltzmann law, which states that the radiant energy equals the emissivity times the Stefan-Boltzmann constant (5.7.times.10.sup.-8 Wm.sup.-2 K.sup.4) times the temperature to the fourth power.
The hemispherical spectral content of the radiation of a black body can be determined using Planck's radiation formula, namely: ##EQU1## wherein, W.sub..lambda.b =the black body spectral emittance with a spectral interval of 1 .mu.m, W/cm.sup.2 .mu.m
T=absolute temperature, .degree.K. PA1 .kappa.=Boltzman's constant=1.4.times.10.sup.-23 JL.sup.-1 PA1 h=Planck's constant=6.6.times.10.sup.-34 J.multidot.sec PA1 c=velocity of light=3.times.10.sup.8 m/sec. PA1 .epsilon.=emissivity, and PA1 .rho.=reflectivity.
Since the amount of radiation emitted and the spectral distribution of the radiation can be measured, it is theoretically possible to determine the temperature of a body for emissivity .epsilon. from the measured radiation. In practice, however, emissivity is not a known constant and is, in fact, not a constant at all but a function of wavelength and temperature. Accordingly, to use pyrometry to measure temperatures accurately, the system must be calibrated to enable it to compensate for emissivity changes.
There are, however, many problems with this technique which lead to significant inaccuracies. Apart from the possibility of measuring stray radiation from nearby hot spots as well as the intended radiation from the surface under measurement, there is the problem that inaccuracies may arise from spectrally-selective atmospheric absorption. Furthermore, unpredictable variations in surface emissivity may arise due to surface roughness, surface conditions (degree of oxidation, etc.) and variations due to the measurement of different materials.
One attempt to improve accuracy has been to use a dual wavelength radiometer. This arrangement, also known as a ratio pyrometer, measures the quantity of radiation emitted by the surface to be measured at two different wavelengths or wavelength bands. An algorithm ratios the signals from the two wavelengths and determines the temperature accordingly. While this procedure theoretically makes temperature measurement independent of the emissivity of the surface, this would only be the case if the emissivity at both wavelengths were the same or is known to change with a constant relationship. However, this is not the case in practice.
Another attempt to improve accuracy has involved the use of an integrating cavity. This is a concave cavity having a reflective inner surface which is placed close to the surface to be measured. Radiation from an enlarged area of the surface covered by the cavity undergoes multiple reflections before it enters the focal zone of the receptor, thus reducing the sensitivity of the pyrometer to changes in surface emissivity and improving the signal to noise ratio. The integrating cavity also has the additional benefit of shielding the sensor from stray heat sources.
Further improvements have been made by measuring the reflectivity of the surface to be measured by directing infra red-containing light from an incandescent light source onto the surface and measuring the intensity of the reflected light. The measured reflectivity can then be used as an indication of surface emissivity because reflectivity is related to emissivity by the formula: EQU .epsilon.=1-.rho.
wherein:
Consequently, if the reflectivity can be measured for the surface at a particular temperature, the emissivity can be calculated and this can be used to determine the surface temperature with improved accuracy.
If the total reflected radiation from the surface to be measured is collected at the same time as radiation emitted from the surface, a combined signal is produced by the pyrometer. In order to distinguish the component of the combined signal due to the reflected light from the component due to radiation emitted from the surface, a so-called "ripple technique" has been developed (see Schietlinger et. al., technical notes from Material Research Society Spring, 1991 Meeting, the disclosure of which is incorporated herein by reference) in which a rapid variation in intensity of the light due to the use of an alternating current to power the light source is employed as a signature of the reflected component. The ratio of the magnitude of the ripple in the incident light from the source to the magnitude of the ripple detected in the output from the pyrometer indicates the ratio of incident light to reflected light. Hence the reflectivity can be determined and used to provide a value for the emissivity.
Despite these refinements, it is still found that the degree of inaccuracy in the temperature measurements may be as high as .+-.10.degree. C. for commercial pyrometers, which is too high for practical value in many cases, particularly when the temperatures of moving metal surfaces within the range of less than about 400.degree. C. are to be measured.
In such cases, the infra-red radiation emitted from the surface to be measured is quite weak and signal to noise ratios in the outputs from the measuring devices may consequently be quite low. Moreover, the movement of the surface to be measured in one direction causes an incident light reflection problem when the surface is anisotropic, as in the case of rolled metal sheets. Anisotropy may result from the presence of directional surface patterns such that a cross-section of the surface taken in one direction exhibits a different two-dimensional configuration than a cross-section taken at an angle to the first direction (e.g. 90.degree. ). The principal anisotropic features of metal sheets are the rolling lines which run parallel to the sides of the sheets.